1. Field of the Invention
The present invention relates to a fluid pressure release valve and in particular to a pressure release valve in which the opening pressure can be varied.
2. Description of the Prior Art
Fluid pressure release valves are often used as safety valves in equipment in which the presence of fluids at high pressures may be dangerous and which may cause damage to the equipment or the user. Such pressure release valves include some form of movable element which may be biased to close the valve with a predetermined force, the movable element being arranged within the valve such that pressure from the fluid also can exert a force on the element to counter the predetermined force. When this counter force exceeds the predetermined force the element moves to open the valve and allow release of the fluid pressure. Often the means used to bias the head may be chosen to provide a load which may be varied to allow the valve to open at different pressures dependent on the circumstances of its use.
One such pressure release valve is that commonly used in kitchen pressure cookers. Here weights are used to bias a sealing head to close the valve with a force determined by the weights used. Relatively crude control over the setting of the release pressure may be made allowing for the weights to be changed and the arrangement is such that in the absence of any bias or fluid pressure the valve is closed. Springs have also be employed to exert the mechanical closing force on the sealing head. One end of the spring is attaches to the sealing head and the other may be attached to a variable position mounting, such as to the end of a threaded rod.
Setting of the release pressure may be achieved by screwing or unscrewing the rod.
Solenoid valves, in which a solenoidal electromagnet is used to bias a movable element to close a valve, are also well known and are capable of providing a more refined control of the release pressure setting since the current through the solenoid windings can be well controlled. One such valve is described in European Patent 484 684, and is used as a pressure release valve for breathing gas in a mechanical ventilator. This valve has a sealing head which is located toward the end of one arm of a pivoting balance and a movable element, in the form of a piston, is located toward the end of the other arm of the balance. The piston is movable in response to energization of an electromagnet to pivot the balance and bias the sealing head toward the valve opening with a force determined by the current supplied to the electromagnet. Additionally, springs are arranged to cooperate with the piston such that in the absence of supply current the valve is in the open position.
Regardless of whether mechanical, electrical or a combination biasing means is provided in the pressure release valve the known valves have a problem that the operation of the valve itself may generate undesirable pressure oscillations in the equipment in which it is incorporated. This is because as the fluid pressure within the valve increases to overcome the applied bias on the sealing head, the valve opens, the pressure falls and the applied bias causes the valve to close. The pressure of the fluid, however, may still be sufficiently high so that as the valve closes the pressure within the valve increases to re-open the valve, thus pressure oscillations may be generated.